Wind power generation has received a major impetus due to ever increasing demand for energy, depleting fossil fuel reserves and environmental benefits in particular with respect to the emission of greenhouse gases.
A wind turbine comprises a drive train, which includes all mechanical components, which are in a functional view connected upstream with respect to an electric generator being used for converting mechanical energy/power captured from wind into electric energy/power being supposed to be provided to a power grid. Important drive train components are e.g. rotor blade, a hub, a main shaft and, if applicable, a gearbox.
When operating a wind turbine regularly mechanical oscillations within the drive train occur. Such oscillations may cause an out-of-round operation of the electric generator which has a negative impact not only on the lifetime of the wind turbine but also on the electric quality of the energy/power being provided by the wind turbine to a power grid. Such mechanical oscillations may be so called “torque oscillations” and/or “speed oscillations”. A pure torque oscillation is an oscillation, wherein in the absence of any internal movement of at least one component of the drive train a value for the torque within the at least one component varies over time. A pure speed oscillation is any oscillation, wherein in the absence of torque variations the speed of at least a part of a component of the drive train varies over time. The speed may be in particular a rotational speed, however also a translational or a combination of a translational and a rotational speed is possible. Typically, the variations over time occur periodically.
In particular in a variable speed wind turbine, mechanical oscillations of drive train and in particular of the rotor blades at resonance frequencies can be excited by grid voltage dips and/or by wind speed changes. Mechanical oscillations resulting from such perturbations can cause a trip of the wind turbine and sometimes even structure damages of components (in particular rotor blades) of the drive train. Therefore, it is necessary to damp mechanical drive train oscillations in particular in variable speed wind turbines.
A known basic principle for damping such mechanical oscillations is to control the power output of the wind turbine being fed to the power grid at drive train resonance frequencies so that mechanical oscillations at this frequency can be reduced. In this context wind turbine controllers have been proposed, which comprise appropriate electronic hardware and/or software filters being comprised within the controllers. However, when applying this kind of active damping control by means of a dedicated wind turbine controller, power oscillations at the mechanical resonance frequency of components of the drive train are sent to the power grid, which is not preferred in both a steady state operational condition and intermediate operational condition comprising a ride through a grid fault. This has the effect that it is difficult for a wind turbine operator to meet power quality requirements given by the operator of a power grid, which receives electric power from the wind turbine.
There may be a need for an effective method which allows for reducing mechanical oscillations of a drive train of a wind turbine.